const createListNode = arr => {
  function ListNode(val, next) {
    this.val = val === undefined ? 0 : val
    this.next = next === undefined ? null : next
  }
  let oldListNode = null
  for (let index = arr.length - 1; index > -1; index--) {
    const val = arr[index]
    const newListNode = new ListNode(val, oldListNode)
    oldListNode = newListNode
  }
  return oldListNode
}
